With optical elements composed of glass or plastic, the problem often exists that in a moist environment, and in particular, with temperature changes, fog can form on the surface, whereby the transmission of the optical element is impaired.
For preventing fog on the surface of an optical element, the patent documents JP 2001/097744 A and EP 0782015 A2 disclose the application of a fog reducing polymer layer (anti-fog layer) on the optical element.
Fog reducing polymer layers, as a rule, contain strong hydrophilic polymers, which can absorb water, so that fine water droplets that develop with the condensation of water are absorbed by the layer within a short time.
Along with avoiding fog, with optical elements it is also desirable to reduce the reflection of the surface in order to achieve better transparency. For this, typically one or more thin antireflection layers are applied onto the optical element; the reflection reducing effect of the layers is based on optical interference.
An alternative method for reducing the reflection of plastic substrates is disclosed in German patent document DE 102 41 708 B4. With this method, a nanostructure is created on a surface of a substrate, composed of plastic, by means of a plasma etching process. Here, the nanostructure is produced by bombarding the substrate surface with energy rich ions, which are generated by means of a plasma ion source.
In order to reduce the fog as well as the reflection of the surface of an optical element, European document EP 0871046 A1 proposes to apply a porous oxide layer onto the fog reducing polymer layer; the thickness of the layer is selected to reduce reflection.
Furthermore, U.S. Patent Application Publication 2003/0030909 A1 proposes embedding a thin oxide layer between a comparatively thick fog reducing polymer layer and an additional thin fog reducing polymer layer. In both proposed solutions, an antireflection effect is achieved by means of the interference at the thin layers.
However, the application of one or more additional layers onto a fog reducing polymer layer can have the disadvantage that the water absorption of the polymer layer is impaired. Furthermore, with such an interference layer system, the problem can occur that optimal antireflection is not attained or undesired color impressions occur in the case of oblique light incidence.